If one thinks oneself in the platform of spirit, he will say he is jiva or servant of Krishna. What if one thinks oneself in terms of subtle body?

Thinking in terms of subtle body means identifying oneself with the mind and intelligence. For example one thinks: 'I am smart', 'I am stupid', 'I am a cheerful person', 'I am a very irritable person', 'I like to be active', 'I am an extrovert/introvert' and so on. These are not the qualities of the soul, but temporary characteristics acquired by the subtle body in association with modes of nature. But one begins to think of one's identity in terms of these acquired natures.

Q. Does the soul have any feelings like pain and happiness? If so, is that due to mind, intelligence and false ego covering it? Does the soul feel pain when it leaves the body?

Consciousness is concomitant with the soul, meaning the soul does experience all varieties of feelings such as happiness, disappointment, anger, frustration etc – but not in relation to the gross or subtle body. The feelings experienced in the spiritual world are all transcendental, and are just different flavors of ananda (transcendental bliss).